Download Chromosomes & Inheritance

Chromosomes & Inheritance
Chapter 13 Raven & Johnson
Chapter 15 Campbell
Mendel & Genes
• 1902 – Chromosomal Theory of
Inheritance
• Genes have specific loci on
chromosomes – chromosomes divide
and move
Thomas Hunt Morgan’s work
• Drosophila melanogaster (fruit fly)
• Normal phenotype – wild type – red
eyes
• Mutant phenotype –
white eyes
• Discovered through
mating that white eyes was sex linked
• Gene located on sex chromosome
Thomas Hunt Morgan’s Work
Thomas Hunt Morgan’s work
• If on X chromosome,
females need two
copies to express
recessive gene
• Males need only
one (they are XY)
• Females with one –
carrier (can pass on)
Thomas Hunt Morgan’s work
• Linkage – genes
close to each
other tend to be
inherited together
Genetic Recombination
• Production of offspring
with new combinations
of traits inherited
from two parents
• If unlinked, there
is a 50% frequency
of recombination
Completely linked have no
recombinants –
0% frequency
If normally linked, then
crossing over could break the
linkage and raise the
frequency
Genetic Mapping
• Alfred Sturtevant
• The farther apart two genes are on a
chromosome, the more chances that
crossing over can occur and the
higher the recombinant frequency will
be
• Can use frequency numbers to map
the distance and order of genes –
linkage map
Genetic Mapping
• Testcross design to map the relative
position of three fruit fly genes, body color
(b), wing size (vg), and eye color (cn).
• The recombination frequency between cn
and b is 9%.
• The r.f. between cn and vg is 9.5%.
• The r.f. between b and vg is 17%.
Genetic Mapping
• The only possible
arrangement  eye color gene
between the other two
• Map unit – distance between genes
(1 m.u. = 1% recombination
frequency)
• Distances are added
Sex Chromosomes
• XX – female
• XY – male
• Determined by sperm (X or Y)
• Sex linked – usually on X & recessive
• Fathers pass to all daughters but no sons
• Mothers can pass to either
Sex Chromosomes
• Ex - colorblindness
Sex-linked Disorders
• Dechenne muscular dystrophy
• Progressive muscle weakness
• Missing protein (dystrophin)
• Hemophilia
• Absence of protein
for blood clotting
X-Inactivation
• Females have two X chromosomes, but
one inactivates during embryo
development
• Condenses into Barr body
• So only one X functions in males AND
females
• Occurs when –CH3 groups are added to
cytosine
• Gene on inactive X makes this happen
• A lot more to figure out
Chromosome Errors
• Nondisjunction – homologous
chromosomes or sister chromatids do not
split evenly into gametes
• Result – some have more, some have
less
• Abnormal chromosome # - aneuploidy
• Extra copy – trisomic
• 1 copy missing – monosomic
• Extra full sets – polyploidy
(occurs in plants)
Chromosome Errors
• Deletion – missing part
• Duplication – extra piece
Chromosome Errors
• Inversion – reversal
• Translocation – out of order
Resulting Problems
• Many result in spontaneous abortion
• Down syndrome
• Trisomy 21
• Specific
symptoms
• Correlation
with maternal age
• Result of nondisjunction during meiosis I
Resulting Problems
• XXY – Kleinfelter’s syndrome
• XO – Turner syndrome
• Deletion at #5 – cri du chat
• Translocations – some cancers
• Fragile X – abnormally thin section of
DNA on X chromosome
• Symptoms are different if from sperm or
egg (worse if from egg)
FYI…Cytoplasmic genes
• Genes found on DNA of mitochondria
and chloroplasts (not nucleus) do not
follow all the rules